This invention relates to the characterization of structures and, more particularly, to an imaging system for obtaining carrier profiles of regions in a semiconductor device.
In an article entitled xe2x80x9cJunction Delineation of 0.15 xcexcm MOS Devices Using Scanning Capacitance Microscopyxe2x80x9d by R. N. Kleiman, M. L. O""Malley, F. H. Bauman, J. P. Gamo and G. L. Timp, IEDM Technical Digest, pages 691-694, 1997, scanning capacitance microscopy (SCM) is described as a technique for characterizing the cross-sectional doping profile of semiconductor structures such as metal-oxide-semiconductor (MOS) transistor devices. Such characterization provides important information for device development and for optimizing the process sequence utilized to make the devices.
Conventional SCM imaging involves positioning a probe tip at successive spaced-apart locations on the surface of a sample. At each position, a measurement is made of the change in capacitance that occurs as the bias voltage applied between the tip and sample is varied. By plotting this quantity (dC/dV) as a function of probe position, it is possible to obtain an indication of the doping profile in the sample, as is well known.
Heretofore, SCM imaging of semiconductor samples has been carried out using small-diameter metal-coated probe tips such as cobalt-silicide-coated tips. In practice, metal-coated tips having a diameter as small as about 30-to50 nanometers (nm) have been employed. By utilizing such small-diameter tips, it is possible to resolve channels of devices with gate lengths as small as approximately 150 nm in MOS devices.
Unfortunately, however, the results typically obtained in SCM imaging with metal-coated tips depend strongly on the value of the bias voltage applied between the tip and the sample. In particular, due to the interaction of the metal-coated tip with the sample, specifically in the built-in depletion region associated with the p-n junctons in the sample, the locations of the junctions move as the applied bias voltage is varied. This considerably complicates the interpretation of the doping information obtained during scanning. Moreover, the resolution and contrast obtainable with metal-coated tips are often less than that required for characterizing ultra-small devices.
Accordingly, continuing efforts have been directed by workers skilled in the art aimed at trying to improve SCM-type imaging of semiconductor devices. In particular, these efforts have been focussed on attempting to improve the resolution and contrast of the SCM images and, further, on attempting to locate the p-n junctions in the device in a simple and accurate manner. It was recognized that these efforts, if successful, could significantly enhance the value of SCM-type imaging as an important tool for characterizing the carrier profile of semiconductor devices.
In accordance with the principles of the present invention, a probe tip made of a material that is capable of exhibiting depletion effects is utilized as an active dynamic element for SCM-type imaging of a sample. The concentration of carriers in the tip is selected to be on the order of or less than the highest concentration of carriers in any sample portion to be probed.
In one particular illustrative embodiment of the present invention, an ultra-small probe tip made of a doped semiconductor material is utilized as an active dynamic element for SCM-type imaging of a semiconductor device. By controlling the doping and bias voltage of the probe tip relative to the device, an accurate high-resolution high-contrast carrier profile of the device is obtained. Importantly, the inventive technique is capable of imaging both semiconducting and non-semiconducting (insulating and metallic) regions of a device structure. These unique capabilities of the invention stem from carrier depletion effects that are selectively controlled to occur in the doped probe tip.
In one specific illustrative embodiment of the invention, a positively-biased n-doped silicon probe tip is step-wise scanned over the cross-section of an n-p-n MOS device while the bias voltage applied between the probe tip and the device is varied. At each step of the scan, a value of dC/dV is measured. These measurements are representative of the cross-sectional carrier profile of the device.
In another specific embodiment of the invention, a negatively-biased p-doped silicon probe tip is step-wise scanned in a similar fashion over the cross-section of a p-n-p MOS device to obtain a representation of the carrier profile of the device.